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Employee Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended 122 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Jun. 30, 2017
May 03, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
401(k) plan contribution expense $ 2.0 $ 2.0      
Common Stock Shares issued under ROI Unit Stock Plan         1,271,009
ROI Shares issued to IBG LLC         1,250,000
2007 SIP shares issued under ROI Unit Stock Plan         21,009
Fair value ROI Treasury Stock         $ 38.0
Common stock to be distributed to former employees under the ROI Unit Stock Plan 3,935        
Maximum shares of stock distributable under 2007 Stock Incentive Plan 30,000,000     30,000,000  
Shares granted to external directors       22,996  
Stock Incentive Plan Granted Shares Adjustment     5,657    
2007 Stock Incentive Plan Compensation Expense $ 25.0 $ 22.0      
Estimated Future 2007 Stock Incentive Plan Compensation Expense $ 30.0     $ 30.0  
Post employment shares distribution       564,825  
2007 Stock Incentive Plan (Shares)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting Percentage • 10% on the first vesting date, which is on or about May 9 of each year; and• an additional 15% on each of the following six anniversaries of the first vesting, assuming continued employment with the Company and compliance with non-competition and other applicable covenants.